Water is an essential component for living organisms on earth and it is regarded as a symbol of pureness and cleanness. 71% of the surface of earth is covered with water and 65% of a human body is composed of this simple yet fully active molecule. In nature, especially in a biological world, water plays an important role. As such, if a water-retaining material with high water retention rate is produced, it can be used as an important material having a positive effect on environment and broad spectrum of use. Until now, an attempt to produce a water-retaining material with high water retention rate was not so successful due to poor mechanical strength as easily expected. However, focusing on a potential possibility of using a material containing water as a main component, for drug delivery or an artificial cartilage, and the like, several research groups succeeded in improving the mechanical strength of a material (Non-patent Documents 1 to 3).
The polymer hydrogel conventionally known as a water-retaining material is basically produced by crosslinking based on covalent bond. The polymer hydrogel that is crosslinked by a conventional method of organic chemistry is an opaque and soft material with low strength and also it has no self-restoring property.
To improve the strength of a hydrogel, an organic and inorganic hybrid hydrogel which has improved tensile fracture strength by a method of polymerizing (meth)acrylamide derivatives in the presence of clay minerals is suggested (Patent Document 1). Furthermore, it is also reported that the strength is improved by having a copolymer with a polymerizable monomer having a carboxylic acid group or a sulfonic acid group (Patent Document 2).
Furthermore, a polymer/inorganic hybrid nano composite hydrogel wherein a polymer is composited with water-swellable clay minerals is now drawing attention as it exhibits relatively good mechanical strength. However, the water retention rate of these gels is only 90% and the mechanical strength decreases as the water retention rate increases. Thus, to obtain practically meaningful strength, it is necessary to decrease the water retention rate to 80% or so (Non-patent Document 4).
As polymerization or crosslinking is required to produce conventionally known polymer hydrogels, it is necessary to perform a complicated process such as repeated heating or cooling cycle. For example, it is reported that a hydrogel can be obtained by dehydration and reswelling of an agglomerate which is obtained by maintaining polyvinyl alcohol with saponification degree of 90% or higher under a pressure of 1.2 to 5.0 atm at 105 to 150° C. in the presence of steam (Patent Document 3).
Such hydrogel is used in many areas of industry including pharmaceuticals, food products, cosmetics, hygienic products, agricultural materials, electronic materials, and the like. For example, there is a pharmaceutical agent obtained by containing GM-CSF in a hydrogel made of polylactic acid and the like (Patent Document 4), a joint supplemental preserving material containing a photo-crosslinkable and biodegradable hydrogel using oligo (poly(ethylene glycol)fumarate) (Patent Document 5), a biomaterial having a morphology developing property on an account of a polymer-crystal based hydrogel of a polymer having a linear polyethyleneimine skeleton (Patent Document 6), a polymer hydrogel electrolyte for an alkaline battery made of polyvinyl alcohol, an anionic crosslinked polymer, and alkali hydroxide (Patent Document 7), and a hydrogel comprising an ionic organic compound having a quaternary ammonium cation which is formed from a heterocyclic compound such as pyridine (Patent Document 8), and the like.
For an application to pharmaceuticals and food products, and the like, human safety is required. Further, from the viewpoint of environment protection, biodegradability is required. As such, it is desired for the hydrogel-constituting materials to satisfy such requirements. For example, it is known that polyethylene glycol or a certain kind of polyester is readily discharged from a human body or it is biologically easily degradable (Non-patent Documents 5 and 6). It is also known that a clay nano sheet, which has as a source material inorganic minerals that are present in nature, is a safe material widely used in cosmetics such as lotion, tooth paste, shampoo, shower gel, and the like (Non-patent Document 7). In this connection, development of a hydrogel using these materials is waited for.
Meanwhile, “dendrimer” indicates a tree-shaped polymer having a regularly branched structure stemming from a core, and the name comes from the Greek word meaning a tree. A dendrimer is composed of a center molecule called core and a side chain called dendron. The number of the repeating branching cycles in a dendron part is described as generation. In general, a polymer has a certain molecular weight distribution, and a dendrimer of higher generation has a unique characteristic that it has almost single molecular weight even though the molecular weight is close to several tens of thousands. As the core is covered with dendron and in an environment isolated from the outside, it is found to exhibit a unique light-emitting behavior or reactivity, and therefore its use as a new functional material is expected. However, as the synthesis is very difficult compared to other polymers, it is not practically usable yet. The PAMAM dendrimer, and the like, which has a polyamidoamine structure and is now used widely, is commercially obtainable from a drug company (source: free encyclopedia “Wikipedia”).
Until now, no application of a dendrimer on hydrogel is made. However, application on DDS and the like is expected. Furthermore, it is reported that an ionic dendrimer in which a cation or an anion such as ammonium ion and carboxylate ion is bonded to the surface of a dendrimer such as PAMAM dendrimer is useful as an anti-parasitic composition (Patent Document 9).